Fix auth bypasses, race condition, stale connections, and unbounded query

- Block disabled accounts at login and unimpersonate (C4, C2)
- Catch IntegrityError on register commit to return 409 instead of 500 (C5)
- Stop _seed_admin from overwriting existing admin password on restart (C8)
- Cap public/stats query at 500 users to bound memory usage (C3)
- Rate-limit PUT /me/password at 5/minute (C6)
- Bump SQLAlchemy to 2.0.50 and aiomysql to 0.3.2; re-enable pool_pre_ping (C7)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/main.py

@@ -33,8 +33,7 @@ async def _seed_admin() -> None:
                 display_name="Admin",
                 is_admin=True,
             ))
-        else:
-            user.password_hash = hash_password(settings.admin_password)
+        elif not user.is_admin:
             user.is_admin = True
         await db.commit()